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Summary Of Investments Other Than Investments In Related Parties (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | $ 3,264,525 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost | $ 3,062,861 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost
Fair value | 3,403,822 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue | 3,158,312 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue
Amounts in the balance sheet | 3,403,822 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount | 3,158,312 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount
Fixed Maturity Securities [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 2,503,494 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember | 2,523,042 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember
Fair value | 2,618,400 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember | 2,560,653 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember
Amounts in the balance sheet | 2,618,400 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember | 2,560,653 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember
Fixed Maturity Securities [Member] | U.S. Government Bonds And Agencies [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 16,028 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 15,994 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fair value | 16,108 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 16,096 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Amounts in the balance sheet | 16,108 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 16,096 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fixed Maturity Securities [Member] | Municipal Securities [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 2,160,710 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 2,201,047 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Fair value | 2,275,455 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 2,235,323 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Amounts in the balance sheet | 2,275,455 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 2,235,323 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Fixed Maturity Securities [Member] | Mortgage-Backed Securities [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 45,519 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 37,848 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Fair value | 47,691 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 40,247 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Amounts in the balance sheet | 47,691 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 40,247 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Fixed Maturity Securities [Member] | Corporate Securities [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 258,940 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 264,172 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Fair value | 256,930 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 264,685 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Amounts in the balance sheet | 256,930 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 264,685 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Fixed Maturity Securities [Member] | Collateralized Debt Obligations [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 22,297 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember | 3,981 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember
Fair value | 22,216 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember | 4,302 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember
Amounts in the balance sheet | 22,216 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember | 4,302 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember
Equity Securities [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 387,851 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember | 223,933 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember
Fair value | 412,880 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember | 281,883 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember
Amounts in the balance sheet | 412,880 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember | 281,883 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember
Equity Securities [Member] | Non-Redeemable Preferred Stock [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 28,012 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_NonredeemablePreferredStockMember | 29,740 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_NonredeemablePreferredStockMember
Fair value | 28,563 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_NonredeemablePreferredStockMember | 29,567 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_NonredeemablePreferredStockMember
Amounts in the balance sheet | 28,563 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_NonredeemablePreferredStockMember | 29,567 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_NonredeemablePreferredStockMember
Equity Securities [Member] | Partnership Interest in a Private Credit Fund [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 10,000 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mcy_PrivateCreditFundMember | 10,000 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mcy_PrivateCreditFundMember
Fair value | 11,719 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mcy_PrivateCreditFundMember | 12,548 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mcy_PrivateCreditFundMember
Amounts in the balance sheet | 11,719 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mcy_PrivateCreditFundMember | 12,548 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mcy_PrivateCreditFundMember
Equity Securities [Member] | Public Utilities [Member] | Common Stock [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 96,482 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / mcy_CommonStockTypeAxis = mcy_PublicUtilitiesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 81,128 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / mcy_CommonStockTypeAxis = mcy_PublicUtilitiesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Fair value | 105,485 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / mcy_CommonStockTypeAxis = mcy_PublicUtilitiesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 85,287 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / mcy_CommonStockTypeAxis = mcy_PublicUtilitiesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Amounts in the balance sheet | 105,485 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / mcy_CommonStockTypeAxis = mcy_PublicUtilitiesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 85,287 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / mcy_CommonStockTypeAxis = mcy_PublicUtilitiesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Equity Securities [Member] | Banks, Trust, And Insurance Companies [Member] | Common Stock [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 7,701 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / mcy_CommonStockTypeAxis = mcy_BanksTrustAndInsuranceCompaniesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 1,610 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / mcy_CommonStockTypeAxis = mcy_BanksTrustAndInsuranceCompaniesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Fair value | 9,757 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / mcy_CommonStockTypeAxis = mcy_BanksTrustAndInsuranceCompaniesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 2,927 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / mcy_CommonStockTypeAxis = mcy_BanksTrustAndInsuranceCompaniesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Amounts in the balance sheet | 9,757 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / mcy_CommonStockTypeAxis = mcy_BanksTrustAndInsuranceCompaniesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 2,927 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / mcy_CommonStockTypeAxis = mcy_BanksTrustAndInsuranceCompaniesMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Equity Securities [Member] | Energy and Other [Member] | Common Stock [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 245,656 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / mcy_CommonStockTypeAxis = mcy_EnergyAndOtherMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 101,455 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / mcy_CommonStockTypeAxis = mcy_EnergyAndOtherMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Fair value | 257,356 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / mcy_CommonStockTypeAxis = mcy_EnergyAndOtherMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 151,554 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / mcy_CommonStockTypeAxis = mcy_EnergyAndOtherMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Amounts in the balance sheet | 257,356 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / mcy_CommonStockTypeAxis = mcy_EnergyAndOtherMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 151,554 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / mcy_CommonStockTypeAxis = mcy_EnergyAndOtherMember / us-gaap_DebtSecurityAxis = us-gaap_EquitySecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Short-Term Investments [Member] |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items] |  |
Cost | 373,180 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember | 315,886 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember
Fair value | 372,542 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember | 315,776 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember
Amounts in the balance sheet | $ 372,542 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember | $ 315,776 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_ShortTermInvestmentsMember

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